My son was admitted off the waitlist in 2013. In his essay he wrote things that he had done since he had submitted the essay. He got all A’s first semester (in 5 AP classes), received some awards, got a sailing instructor license, etc… He was admitted first wave off the waitlist (but ultimately decided to go to UCB, which had given him a Regents Scholarship). So I would definitely recommend updating them with all the fabulous things you have done since you originally submitted your application.